The present invention relates generally to heat-transfer labels and more particularly to a novel heat-transfer label including a polyester ink layer.
Heat-transfer labels are commonly used in the decorating and/or labelling of commercial articles, such as, and without limitation to, containers for beverages (including alcoholic beverages such as beer), essential oils, detergents, adverse chemicals, as well as health and beauty aids. As can readily be appreciated, heat-transfer labels are desirably resistant to abrasion and chemical effects in order to avoid a loss of label information and desirably possess good adhesion to the articles to which they are affixed.
One well-known type of heat-transfer label is described in U.S. Pat. No. 3,616,015, inventor Kingston, which issued October, 1971, and which is incorporated herein by reference. In the aforementioned patent, there is disclosed a heat-transfer label comprising a paper sheet or web, a wax release layer affixed to the paper sheet, and an ink design layer printed on the wax release layer. In the heat-transfer labelling process, the label-carrying web is subjected to heat, and the label is pressed onto an article with the ink design layer making direct contact with the article. As the paper sheet is subjected to heat, the wax layer begins to melt so that the paper sheet can be released from the wax layer. After transfer of the design to the article, the paper sheet is immediately removed, leaving the design firmly affixed to the article and the wax layer exposed to the environment. The wax layer is thus intended to serve two purposes: (1) to provide release of the ink design from the web upon application of heat to the web and (2) to form a protective layer over the transferred ink design. After transfer of the label to the article, the transferred wax release layer is typically subjected to a post-flaming technique which enhances the optical clarity of the wax protective layer over the ink design and which enhances the protective properties of the transferred wax release.
In some heat-transfer labels, an adhesive layer is deposited over the ink design to facilitate adhesion of the label onto a receiving article. An example of such a heat-transfer label is disclosed in U.S. Pat. No. 4,548,857, inventor Galante, which issued Oct. 22, 1985, and which is incorporated herein by reference. Additionally, in some heat-transfer labels, a protective lacquer layer is interposed between the wax release layer and the ink layer. An example of such a label is disclosed in U.S. Pat. No. 4,426,422, inventor Daniels, which issued Jan. 17, 1984, and which is incorporated herein by reference.
One problem that has been noted with heat-transfer labels of the type described above containing a wax release layer is that, quite often, a degree of hazing or a "halo" is noticeable over the transferred label when the transfer is made onto clear materials. This "halo" effect, which persists despite post-flaming, is a result of the nature of the wax coating around the outer borders of the transferred ink design layer. Hazing due to the wax release layer may also appear in "open-copy" areas of the label, i.e., areas of the label where no ink is present between the adhesive and protective lacquer layers, and also represents a problem.
Accordingly, to overcome the aforementioned "halo" effect, considerable effort has been expended in replacing or obviating the need for a wax release layer. One such wax-less, heat-transfer label is disclosed in U.S. Pat. No. 3,922,435, inventor Asnes, which issued Nov. 25, 1975. In the aforementioned patent, the layer of wax is replaced with a layer of a non-wax resin. This non-wax resinous layer is referred to in the patent as a dry release since it does not transfer to the article along with the ink design layer. In a preferred embodiment of the patent, the non-wax resinous layer comprises a thermoset polymeric resin, such as cross-linked resins selected from the group consisting of acrylic resins, polyamide resins, polyester resins, vinyl resins and epoxy resins.
Another example of a wax-less, heat-transfer label is disclosed in U.S. Pat. No. 4,935,300, inventors Parker et al., which issued Jun. 19, 1990, and which is incorporated herein by reference. In the aforementioned patent, the label, which is said to be particularly well-suited for use on high density polyethylene, polypropylene, polystyrene, polyvinylchloride and polyethylene terephthalate surfaces or containers, comprises a paper carrier web which is overcoated with a layer of polyethylene. A protective lacquer layer comprising a polyester resin and a relatively small amount of a nondrying oil is printed onto the polyethylene layer. An ink design layer comprising a resinous binder base selected from the group consisting of polyvinylchloride, acrylics, polyamides and nitrocellulose is then printed onto the protective lacquer layer. A heat-activatable adhesive layer comprising a thermoplastic polyamide adhesive is then printed onto the ink design layer.
Although the above-described wax-less, heat-transfer label eliminates the waxrelated "halo" effect discussed previously, said label does not quite possess the same release characteristics of heat-transfer labels containing a wax release layer. Accordingly, another type of heat-transfer label differs from the heat-transfer label disclosed in U.S. Pat. No. 4,935,300, only in that a very thin layer or "skim coat" of a waxlike material is interposed between the polyethylene release layer and the protective lacquer layer to improve the release of the protective lacquer from the polyethylene-coated carrier web. The thickness of the skim coat corresponds to approximately 0.1-0.4 lbs. of the waxlike material spread onto about 3000 square feet of the polyethylene release layer.
The aforementioned types of heat-transfer labels may be used to decorate a variety of surfaces and materials including, but not limited to, glass and polyethylene terephthalate (PET) containers. Glass containers are frequently, although not invariably, pre-treated (typically by the container manufacturer) with polyethylene, oleic acid, stearate or a similar material whose function is to enhance abrasion resistance and lubricity. Such containers, whether or not previously subjected to the foregoing type of pre-treatment (or whether or not such a pre-treatment is later removed from the container), are typically treated, prior to decoration, with a silane adhesion promoter of the type described in U.S. Pat. No. 3,907,974, inventor Smith, which issued Sep. 23, 1975 and which is incorporated herein by reference.
One type of polyethylene terephthalate container is referred to in the art as a refundable polyethylene terephthalate (RefPET) container. A RefPET container, which typically has a thickness greater than a non-refundable PET container, is intended for one or more re-uses after its initial use and is designed to withstand caustic washings between uses.
An example of a heat-transfer label which has been used by the assignee of the present application on refundable polyethylene terephthalate (RefPET) containers comprises a support portion in the form of polyethylene-coated paper and a transfer portion including a protective lacquer layer printed onto the polyethylene-coated paper and comprising a polyester/vinyl lacquer and a wax, a polyester/vinyl ink design layer printed on the protective lacquer, and a polyester adhesive printed on the polyester/vinyl ink design layer. A 4:1 mixture of ViTEL.RTM. 2300 polyester resin (Shell Chemical Company, Akron, Ohio): VAGH vinyl resin (Union Carbide Corporation, Hackensack, N.J.), with a suitable colorant, in 1:1 toluene:methyl ethyl ketone (MEK) is one example of a polyester/vinyl ink system used in the aforementioned label.
One type of ink design commonly employed in heat-transfer labels of the type described above is referred to in the art as "line copy," and another type of ink design commonly employed in such labels is referred to in the art as a "vignette." Line copy typically refers to an ink design generated by complete coverage of a desired label area with ink. A vignette typically refers to an ink design characterized by a gradual change in color from a first color to a second color and generated by the arrangement of a plurality of ink dots of varying size and color.
With respect to the aforementioned RefPET heat-transfer labels, the present inventor has observed that the ink vehicles (i.e., the inks without pigments) tend to be hazy, thereby indicating an incompatibility of the resins in the ink. In addition, at constant solids, the viscosity of the vehicles rapidly increase with decreasing amounts of the more volatile MEK. In actual printing, the more volatile solvent leaves first, and solids increase due to evaporation. All this leads to very rapid increases in viscosity. Moreover, the supplier of the polyester resin reports that a toluene/MEK solvent system of the type being used is not the best solvent system for this resin. The net effect of the problems described above is that yields in the manufacturing process have been less than optimal. Moreover, when the present inventor has tried to use the foregoing polyester/vinyl ink to print vignettes on the above-mentioned protective lacquer, printing quality has been rather poor and yields have been quite low.
Glass containers have also been labelled by the assignee of the present application using heat-transfer labels similar to those used in labelling RefPET containers, the heat-transfer labels for glass containers comprising polyethylene-coated paper, a wax-like skim coat overcoated on the polyethylene-coated paper, a protective lacquer printed onto the skim coat and comprising a polyester/vinyl lacquer, a polyester/vinyl ink design layer printed on the protective lacquer, and a polyester adhesive printed on the polyester/vinyl ink design layer.